Transistors are semiconductor devices that are commonly found in a wide variety of integrated circuits. A transistor is basically a switch. When a voltage is applied to a gate of the transistor that is greater than a threshold voltage, the switch is turned on, and current flows through the transistor. When the voltage at the gate is less than the threshold voltage, the switch is off, and current does not flow through the transistor.
Traditionally, the gates of transistors have been polysilicon gates. Transistors having polysilicon gates are relatively easy to fabricate, and the operating effects of transistors having polysilicon gates are well known. However, as power consumption and operating speeds of integrated circuits that include transistors are optimized, more recently the gates of transistors have been metal gates.
A transistor having a metal gate can be fabricated in two general ways. A polysilicon gate can first be fabricated, and then replaced with a metal gate during subsequent semiconductor processing. This approach is known as a “gate last” approach, and the metal gate is considered a damascene or replacement gate, insofar as the metal gate replaces a polysilicon gate. A second approach fabricates the metal gate without first fabricating a polysilicon gate, and is known as a “gate first” approach.